1. Field of the Invention
The present invention relates to imaging lens systems. More particularly, the present invention relates to an imaging optical device that acquires an image of a subject by use of an image sensing device (for example, a solid-state image sensing device such as a CCD (charge-coupled device) image sensor or a CMOS (complementary metal-oxide semiconductor) image sensor), a digital device that incorporates such an imaging optical device so as to be equipped with an image input function, and a compact imaging lens system that forms an optical image of the subject on the light receiving surface of the image sensing device.
2. Description of Related Art
It is today common to incorporate an imaging optical device employing a solid-state image sensing device such as a CCD image sensor or a CMOS image sensor in portable terminals. With the spread of such portable terminals, with a view to obtaining higher-quality images, portable terminals incorporating imaging optical devices employing image sensing devices with increasingly large numbers of pixels are being supplied to the market. While, conventionally, image sensing devices with large numbers of pixels are accordingly large in size, in recent years, as pixels are made increasingly fine, image sensing devices have come to be made compact. Imaging lens systems that are used with image sensing devices with fine pixels are required to have high resolution proportionate to the fineness of pixels.
The resolution of a lens is limited by its f-number, and the smaller the f-number, that is, the faster the lens, the higher resolution can be obtained, hence the demand for fast imaging lens systems. On the other hand, to make imaging optical devices more compact, imaging lens systems are required to have shorter total lengths. Elaborating the power arrangement, lens thicknesses, and aerial distances does make imaging lens systems more compact, but not beyond a certain limit. In recent years, therefore, there have been made attempts to shorten the total length of the optical system by use of a wide-angle lens system, that is, an imaging lens system with a shorter focal length. As imaging lens systems for such uses, there have been proposed imaging lens systems of a five-element design for the reason that they can be adapted for higher performance than those of a three- or four-element design.
As such imaging lens systems of a five-element design, Patent Documents 1 and 2 disclose imaging lens systems that are composed of, from the object side, a first lens element having a positive optical power, a second lens element having a negative optical power, a third lens element having a positive optical power, a fourth lens element having a positive optical power, and a fifth lens element having a negative optical power.
Patent Document 1: Japanese Patent Application Publication No. 2010-262269
Patent Document 2: Japanese Patent Application Publication No. 2012-8164
Inconveniently, however, with the imaging lens system disclosed in Patent Document 1, the interval between the second and third lens elements is so short that the beam that has exited from the second lens element enters the third lens element before diverging sufficiently, and this makes it impossible to sufficiently correct the coma aberration occurring in the second lens element. With the imaging lens system disclosed in Patent Document 2, the object-side curvature of the third lens element is so small that it is impossible to sufficiently correct the coma aberration occurring in the off-axial beam.